The present invention relates to a method and circuit for saving the power of a battery used in a portable cellular radio telephone.
Generally the portable cellular radio telephone includes elements using a low operating current to save the power of a battery. Moreover, it is so made as to intermittently receive the power supply of the battery only required for a signal reception during the idle state. Such technique is widely used in mobile radio telecommunication units like a paging system, which technique is disclosed in Korean Patent Application No. 89-1749 of the present applicant and U. S. Pat. No. 4,479,261 of Oda.
Another kind of such technique is disclosed in U.S. Pat. No. 4,903,319 entitled as "Portable Radio Telephone Having Power Saver". In this case, a portable radio telephone is made so that other elements except a processor are cut off from the battery power in the outside of the network service area. Referring to FIG. 1, a comparator 28 compares a received signal strength indicator signal RSSI with a threshold voltage supplied from a voltage source 29. This threshold voltage represents the signal level at the boundary of the service area. When the portable radio telephone is outside of the service area, the RSSI signal is lower than the threshold and the comparator 28 generates a low voltage output applied to the processor 13 to cut off the battery power from supplying.
However, the processor 13 is continuously supplied with the battery power. Moreover the RSSI signal changes too abruptly due to fading to be used for judging whether the portable radio telephone is within the network service area, and it is very difficult to determine the threshold voltage.